Work is continuing on the purification of MAO. If highly purified MAO still contains appreciable amounts of iron the next steps are: 1. To determine which ligands are involved in iron binding. MAO is reported to contain 8 cysteine residues per 100,000 mw, and these appear to be essential for activity. MAO would be titrated with sodium mersalyl to determine whether iron was released in this process, 2. to determine the function of iron - MAO preparations have been reported which have good activity but widely varying amounts of iron. Possibly in vitro assays do not measure the role of iron in this enzyme. For example, iron may participate in electron transfer to an intermediate carrier in the mitochondria rather than to 02.